Is H2S polar or nonpolar? - Polarity of Hydrogen sulfide
H2S is the chemical formula for dihydrogen sulfide, a colorless gas that is released as an industrial by-product and by the decay of organic matter. Even a small concentration of hydrogen sulfide (H2S) in the air can be sensed instantly because it smells like rotten eggs.
This article brings to you the answer to a very important question about the chemical nature of H2S i.e., is it polar or nonpolar?
The answer to this question can be a bit complex sometimes but we will try our best to help you burst this confusion.
Is H2S polar or nonpolar?
Hydrogen sulfide (H2S) is a weakly polar molecule. Its polarity is unique. This is because there is only a slight electronegativity difference between the hydrogen (H) and sulfur (S) atoms bonded in H2S.
Thus, an H-S bond is non-polar, but the molecule overall is polar due to the asymmetric bent shape of the H2S molecule. H2S has a net dipole moment greater than 0.
|Name of molecule||Hydrogen sulfide (H2S)|
|Bond type||Non-polar covalent|
|Molecular geometry||Bent or V-shaped|
|Polar or Non-polar?||Weakly polar molecule|
|Dipole moment||0.95 D|
Why is H2S a polar molecule?
The attractive force between atoms created by an electron pair sharing is called a covalent bond.
There are two types of polarities in covalently bonded molecules namely :
- Bond polarity
- Molecular polarity
If a covalent bond is formed between two dissimilar atoms which have a difference in their electronegativities then such a bond will be a polar covalent bond. This is called bond polarity.
If all the polar bonds present in a molecule are arranged such that there is a non-uniform charge distribution in the molecule overall, then it will be a polar molecule. This is known as molecular polarity.
There are three factors that help in explaining the polarity of H2S namely:
- Dipole moment
- Molecular geometry or shape
Let us discuss all the factors one by one so that you can understand better the polar nature of an H2S molecule.
Factors affecting the polarity of H2S
Electronegativity is defined as the tendency of an atom to attract a shared pair of electrons from a covalent bond.
Electronegativity of elements generally increases as you go from left to right in the Periodic Table while it decreases down a group.
An H2S molecule is made up of two H atoms bonded to an S atom. Sulfur (S) is slightly more electronegative (E.N = 2.58) than hydrogen (E.N = 2.20 ).
Both atoms have an electronegativity difference of about 0.38 units.
|But a bond is considered polar mainly when the electronegativity difference between two bonded atoms is at least greater than 0.5.|
According to this definition, the individual H-S bonds in the H2S molecule are nonpolar.
Technically, H2S should be a non-polar molecule on account of the non-polar bonds present in it. But, this concept is not that simple.
Sulfur though only weakly attracts the shared electron cloud from each H-S bond still, this attraction induces a partial negative (Sδ-) charge on sulfur and a partial positive (Hδ+) charge on each of the bonded hydrogen atoms.
The dipole moment is a vector quantity that represents the polarity of a bond and/or a molecule.
It has a symbol µ and is defined as the product of electrical charge (Q) and charge separation i.e., bond length (r). Debye (D) is the symbol used for dipole moment.
⇒ µ= Q.r
It points from the partial positive center to the partial negative center of a bond or a molecule.
The two charged centers in an H-S bond are Hδ+ and Sδ-. So, the dipole moment points from Hδ+ to Sδ-.
It has a numerical value approximately equal to the electronegativity difference between H and S atoms i.e., 0.38 D.
The sulfur (S) atom belongs to group VII A (or group 16) of the Periodic Table. Therefore, sulfur has 6 valence electrons available for bonding.
Sulfur uses 2 of its valence electrons for forming a single covalent bond with an H-atom on each side while the other 4 electrons are situated as 2 lone pairs on sulfur.
In this manner, S achieves a complete octet electronic configuration while each H atom completes its duplet and attains stability.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, H2S is an AB2E2 type molecule.
When there are four electron pairs around a central atom in a molecule, the ideal electronic geometry of the molecule is tetrahedral.
But in the case of H2S the 2 lone pairs (E) present on sulfur lead to lone pair- lone pair repulsions and lone pair-bond pair repulsions which decrease the H-S-H bond angle.
The bond angle decreases from the ideal 109.5° of a tetrahedron to 92.1° in the presence of lone pairs. Thus, H2S achieves an asymmetric, angular, or bent shape.
The dipole moment effect of individual H-S bonds is enhanced in H2S which makes the overall hydrogen sulfide (H2S) a polar molecule with a net dipole moment µ = 0.95 D.
Difference between polar and non-polar molecules
|Polar molecule||Non-polar molecule|
|Atoms having a difference in|
|Atoms may have the same or different electronegativity values|
|Unequal charge distribution overall||Equal charge distribution overall|
|Net dipole moment greater than 0||Net dipole moment equals to zero|
|Examples include water (H2O), hydrogen sulfide (H2S), ammonia (NH3) and sulfur dioxide (SO2), etc.,||Examples include oxygen (O2), nitrogen (N2), methane (CH4), and carbon dioxide (CO2).|
Also check –
Why H2S is a polar molecule?
Which is more polar, NH3 or H2S?
H2S has non-polar bonds present in it but still, it is a slightly polar molecule because of the asymmetric bent shape of the molecule that leads to a non-uniform charge distribution.
However, NH3 is extremely polar with polar N-H bonds present in it.
NH3 has a trigonal pyramidal shape with a net dipole moment (µ) value of 1.4 D.
While net µ for H2S is only 0.95 D. So, NH3 is definitely more polar than H2S.
Which is more polar, H2O or H2S?
H2O is more polar than H2S. Both molecules contain 2 hydrogen atoms covalently bonded to a group VII A atom. Both have an asymmetric bent shape and geometry.
But the individual H-S bonds in H2S are non-polar due to a small electronegativity difference (0.38 ) between H and S atoms.
The molecule(H2S) is polar overall because of its asymmetric shape.
On the other hand, polar O-H bonds are present in H2O due to the greater electronegativity difference (1.24) between hydrogen and oxygen atoms.
So, water is a highly polar molecule. Electronegativity decreases down the group, that is why sulfur is less electronegative than oxygen.
Why there is no hydrogen bonding in hydrogen sulfide (H2S)?
There are many factors that influence the hydrogen bonding ability of molecules.
One main factor is the electronegativity of the atom bonded to hydrogen. The sulfur atom has a large size and a relatively lower electronegativity value.
Additionally, there is only a small electronegativity difference between an H and an S atom.
S cannot pull the shared electron cloud towards itself to a significant extent.
Thus, no hydrogen bonding in H2S.
Why is H2O a liquid while H2S is a gas?
Out of all the factors, intermolecular forces of attraction also depend on the polarity of molecules.
H2O is more polar than H2S, it has a higher net dipole moment value therefore the oppositely charged Hδ+ and Oδ- end of an H2O are more strongly held together.
H2O molecules also develop hydrogen bonds with each other thus water has a higher boiling point, and it exists as a liquid at room temperature.
H2S molecules are only weakly polar and there are no H-bonds, so it has a lower boiling point and is a gas.
- H2S is a weakly polar molecule.
- H2S is made up of two H-S bonds. Due to a very small difference in electronegativity between the bonded atoms, an individual H-S bond is considered non-polar.
- The asymmetric, bent shape of H2S due to two lone pairs of electrons present on central sulfur makes the overall molecule polar. So, H2S is an exceptional case.
- The individual H-S dipole moments are not canceled, rather these add up to yield a polar H2S molecule with a net dipole moment µ value of 0.95 D.
- The H-S-H bond angle in the H2S molecule is 92.1° while the H-S bond length is 133.6 pm.